1. Field of the Invention
This invention relates generally to a method and device for detecting ground contact of an electric motor-driven jack as the jack is being extended to adjust the attitude of a mobile platform.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Any mobile platform attitude adjustment system that relies on jacks to move a platform into a desired attitude must have a jack drive mechanism for extending the jacks to the ground, adjusting the attitude of the platform, and retracting the jacks when the platform is to be moved to a different location. It is desirable for the jacks to be firmly seated on the ground before allowing such a jack drive mechanism to lift or adjust the attitude of the platform since failure to properly seat the jacks on the ground can result in unstable and potentially dangerous platform positions or attitudes.
It is also desirable that a controller for controlling such a jack drive mechanism have a feedback mechanism that allows the controller to determine when jacks contact the ground. Such a controller determines when jacks are on the ground to ensure that the platform is firmly seated to prevent the jack drive mechanism from significantly elevating any portion of the platform until after all the jacks have been seated or grounded.
For example, U.S. Pat. No. 5,143,386 issued 1 Sep. 1992, to Uriarte, discloses a platform leveling device including a plurality of jacks powered by respective DC electric jack motors. To detect ground contact the Uriarte patent discloses a controller programmed to interpret electrical motor current draw values exceeding a fixed, predetermined current value, as indicating jack ground contact. However, the Uriarte device is unable to recognize and ignore current spikes that exceed the predetermined current value but are unrelated to jack ground contact. Current spikes unrelated to jack ground contact may be caused by such phenomena as motor in-rush, momentary contact with intervening obstructions, impurities in the jack drive mechanism, and clutching. Neither does Uriarte disclose compensation for variations in jack motor power draw.
Variations in jack power draw can occur for a number of different reasons: As a jack ages, wear on its mechanical parts increases, requiring the motor driving the jack to work progressively harder to lift the same load. Other factors include expansion and contraction due to temperature changes, contamination, loss of lubrication, corrosion, and the like. Jack power draw can also vary depending on which portion of a platform the jack is supporting and the location and starting attitude of the platform relative to the ground. The starting attitude relative to the ground can vary greatly between leveling locations and, when it does, it causes the jacks supporting the platform to contact the ground at different times and to divide the load differently. The jack or jacks bearing more load will work harder and draw more power than the others. Jack power draw can also be affected by changes in the amount of peak power available from a battery or batteries that power the motor driving the jack.
U.S. Pat. No. 4,084,830 issued 18 Apr. 1978, to Daniel, Jr. et al., discloses, for each electric jack in a platform leveling system, a ground contact detection switch mounted in a position to be mechanically closed by a pin that is supported in such a way as to move upward into contact with the switch when an associated jack extends into contact with the ground. A controller is connected in an electrical ground contact detection circuit with each ground contact detection switch and is programmed to interpret a closed ground contact detection circuit as indicating that a corresponding jack has contacted the ground. The controller is further programmed to interpret an open ground contact detection circuit as indicating that a corresponding jack is not contacting the ground.
What is needed is an electric jack ground contact detection method and device that can detect jack ground contact by sensing power draw increases rather than requiring mechanically-actuated switches, that can recognize and ignore current spikes that are unrelated to jack ground contact, and that can compensate for variations in jack motor power draw.